[south65ss]

I just started a freshly rebuilt 230 Chevy six and it idles between 750 and 800 RPM with between 12 and 17 in Hg of vacuum. Could the problem be the cam (204in/216ex at 050, .498 in/.530ex lift and 112 LSA) or just timing or cab tuning? At around 2000 RPM it spins smooth with 19 in Hg of vac. Any clue?

 

[cobalt327]

I just started a freshly rebuilt 230 Chevy six and it idles between 750 and 800 RPM with between 12 and 17 in Hg of vacuum. Could the problem be the cam (204in/216ex at 050, .498 in/.530ex lift and 112 LSA) or just timing or cab tuning? At around 2000 RPM it spins smooth with 19 in Hg of vac. Any clue?

The cam will contribute to a lower than stock vacuum reading, and the engine's small displacement makes the cam seem bigger to it.

That said, there shouldn't be a lot of jiggling of the (undamped, automotive diagnostic-type) vacuum gage.

If the gage needle is bouncing back and forth between 12 and 12 in/Hg, this could indicate a problem with the cam or valve sealing. If the head was rebuilt and the seats and valves were refaced, they could be still leaking either because they need more running to break in (although they shouldn't- they should have been checked at the machine shop to have a good seal) or they were improperly machined.

The other problem could be the cam- with the oil today lacking enough zinc-based wear inhibitors, more and more cams are failing during break-in.

 

[techinspector1]

Put some additional initial ignition timing in at the crank and see if it improves. Don't exceed the total (initial and centrifugal) specified for the motor. Alter the centrifugal to limit it if you must, to dial in the total.

 

[south65ss]

Cobalt327: the head was ported and has big springs and stainless valves and the work was done by a good machine shop, so I want to belive the problem is not there. And I did the correct break in procedure for the cam with texaco diesel multigrade oil with Comp Cam break in aditive, and a good amount of moly grease on the lobes. So, any other idea?

Techinspector1: what do you think is a good initial and total advance for this engine? The distributor is a Mallory Optispark and the engine has forged pistons with something like 10 to 1 compression.

Thanks a lot guys!

 

[techinspector1]

I have no idea the octane of the available fuel in Uruguay or how your combination of parts will react to increased spark timing. Just dial in a little more lead at a time and see how the motor reacts.

 

[south65ss]

97 octane here. Thanks a lot!

 

[cobalt327]

Cobalt327: the head was ported and has big springs and stainless valves and the work was done by a good machine shop, so I want to belive the problem is not there.

I understand that you don't want to believe the problem's there. But it still could be.

Are you using a mechanical advance distributor? If so, the weight/spring combo could be right on the verge of advancing, causing the timing to vary at that idle speed. If you suspect this might be the prob., temporarily install a heavy spring on one side and see if the vacuum stabilizes.

TI's advice is sound (as usual), try more advance if your gas will support it. Is the octane measured by MON + RON x .5?

Just as important as timing, is the fuel/air ratio. If it's off by much at idle, you can get erratic readings. But usually NOT a such a variation, like you describe. Bad carburetion will usually just be a low vacuum reading, but steady, for the most part- not quickly jiggling, like w/a bad valve, etc.

 

[south65ss]

What you said about mechanical advance sounds right, `cause when I was checking with the timing light the advance was very fast between 1000 and 2000 RPM and ended just after 2k close to 40�.
The carb is a brand new 390 Holley with minor modifications according to Clifford specs (power valve and jets) so I guess I will only have to set the idle mix.
Also I was checking a Holley manual and I think I put the vac advance hose in the wrong place.

I couldn`t test anything because I had to leave early from the shop but I will try all this in a couple days and post the results here.

Thanks.

 

[south65ss]

No power on launch

The Camaro is finally on the road, but doesn`t run good at all.
The erratic idle is gone, but there`s no power in the low RPM range and it launches like a Toyota (old one). It even misfires but only sometimes and to get it worse the 700R4 freshly rebuilt doesn`t let the engine rev up. It`s shifting too early no matter how I adjust the tv cable. But even shifting manually the low RPM power is very low. On the road it`s another story. With the 3:06 to 1 and the trans in 4th and locked converter it makes around 75 mph at less than 2000 RPM and feels strong. It also destroyed a brand new set of platinum spark plugs on the first few miles. Maybe because of the carb getting flooded several times? The timing is set 20 deg. BTDC and it goes quickly to around 35 at less than 3000 RPM.
What am I doing wrong?
PLEASE HELP!!!!!!!!!!!!!

 

[south65ss]

It pulls decently at around 2500 or 3000 RPM. Any other clue?

 

[techinspector1]

It pulls decently at around 2500 or 3000 RPM. Any other clue?

Confirms what F-BIRD'88 told you in post #10....
"A motor (especially a small cid motor) with a cam in it has less torque than stock in the low rpm range. The torque curve is shifted up in rpm.
You want a higher stall torque converter and higher rear gears to take advantage of the higher rpm power curve."